Do you want markdown editing in content or for editing the theme. If the theme, I'm not sure why. If for content, I think it's a great idea and has been discussed before. It would could very easily be implemented as a tile in mosaic?
This works after I save, but getting a preview is difficult (?).
... or will this be too slow?
Maybe it could be possible to update only on (one or two) newlines (or a shortkey / button )
It would also be nice if the bodyText field used this editor when set to markdown instead of html.
I have had troubles with using the Richtext field, because the rich text behavior always defaults to html (evn if set in control panel to something else). and adding a Richtext field TTW gives errors on save (instead of saving the input, it saves 'did you mean Raw or output)
Is there any point in making a tile?
I have added it as a behaviour, which means one can just add the field to the layout.
(of course, one can just add one (instance), but I dont think many markdown fields in a mosaic layout is a very common use case
You are thinking in terms of content types. Once you switch to mosaic page content is made up of tiles. This is no body anymore. Or rather the body is made up of tiles. So it wouldn't make sense have a single body which is markdown. But it might make sense to have parts of the page being markdown and parts being rich text.